The present invention relates to methods and apparatus for the chemical processing of gas-liquid chemical reactant mixtures, and more particularly to chemical reactors and processes incorporating structured catalysts assembled from catalyst sections of monolithic honeycomb configuration for the processing of such mixtures.
Interest in the use of structured catalysts such as honeycombs for the processing of fluid chemical reactant streams in three-phase (gas-liquid-catalyst) reactions is increasing. Among the several applications being is considered for this technology are those involving co-current gas-liquid flow through the catalyst. In co-current flow both gas and liquid phases of a two-phase gas-liquid feed stream are passed concurrently and in the same direction through the honeycomb catalyst. What is desired in carrying out such a process is to sustain a flow regime which leads to high mass transfer coefficients in the monolithic structure.
One example of the use of structured catalysts in reactors for chemical processing is found in Published European patent application EP 0 226 306. That application describes a packing design for a chemical reactor employing an interlocking array of honeycomb catalysts having potential application for the processing of a variety of chemical feedstocks.
The interlocking catalyst arrangement described in that patent is designed to avoid catalyst-free pathways through the reactor while at the same time permitting some relative movement among the blocks of honeycomb catalyst. Catalyst movement occurs as the result of dimensional changes in the honeycombs and reactor vessel resulting from temperature differentials arising within the reactor during use.
Separate and orderly fluid flow through parallel honeycomb channels in the manner described in this patent has been considered desirable for many chemical reactions, since such flow fixes the distribution of the reactants and maximizes catalyst utilization. However, there are other chemical reactions that may benefit from more turbulent reactant flow. Thus U.S. Pat. No. 5,514,347 discloses honeycomb structures of complex wall configuration designed to increase flow turbulence and the exchange of chemical reactants among the channels of the honeycombs during use.
One disadvantage of complex honeycomb configurations, however, is high cost. Thus an inexpensive design or method for developing appropriate levels of reactant intermixing would be of interest for a number of chemical processing applications. And, such a design or method would be particularly useful if it permitted some degree of control over the extent of reactant redistribution within the catalyst bed, for example, to prevent undesirable feed stream disruptions causing high feed stream flow through some sections of the catalyst bed, with flow starvation and attendant catalyst under-utilization in other sections of the bed.
The present invention provides chemical reactor designs and processes employing structured catalysts of honeycomb shape. The structured catalysts are configured to achieve a controlled reactant feed stream redistribution that has been found quite effective for improving the conversion efficiency of the reactors. The reactors may be used for a variety of applications, but are particularly useful for carrying out three-phase reactions involving the co-current processing of gas-liquid feed streams over solid honeycomb catalysts.
The invention is based in part on the discovery that arranging the honeycomb catalyst packing to provide xe2x80x9cclosedxe2x80x9d channel discontinuities in the honeycomb channels traversing the reactor increases the apparent activity of the catalyst. Closed channel discontinuities are discontinuities substantially free of unchanneled or unguided flow, such as the unguided flow that occurs in open chambers or spaces within conventional catalytic reactors for promoting random feed stream mixing.
Suitable channel discontinuities for the purpose of the invention are provided by rotating or displacing adjacent, flow-connected blocks of catalyst honeycombs having substantially the same cell density so that their channels are not aligned. The degree of rotation or offset will be sufficient to provide channel splitting, i.e., a flow pattern such that reactants exiting a channel in an upstream block will be sub-divided and redistributed among two or more adjoining channels in a downstream block that is flow-connected thereto.
The result of this flow pattern is significantly higher apparent catalytic activity than provided by equivalent volumes of catalyst with no channel discontinuities, or with conventional mixing sections, without significant added cost for the added catalytic activity. A further advantage of the approach is that the costs of added measures to insure channel alignment are entirely avoided. In fact, honeycomb orientations within the bed may be nearly random, since the effectiveness of the invention does not depend on the exact degree of channel misalignment between the upstream and downstream honeycombs.
Accordingly, in a first aspect, the invention includes an improved chemical reactor for treating a gas-liquid feed stream with structured monolithic catalysts of honeycomb configuration. A suitable reactor vessel for the containment of a structured monolithic catalyst bed and the processing of a reactant feed stream is first provided. Included within the reactor are two or more sections of structured honeycomb catalyst, including at least a first catalyst section and a second catalyst section disposed in substantially contacting or otherwise flow-connected end-to-end relationship with each other.
Each of the first and second catalyst sections will have an inlet end and an outlet end between which a plurality of parallel open-ended honeycomb channels bounded by channel walls with catalytically active wall surfaces extend. The channels of both sections are oriented along a common flow axis in a direction of feed stream flow running generally from the inlet port toward the outlet port of the reactor vessel.
To achieve the required controlled feed stream turbulence and mixing, the channels of the first and second catalyst sections are laterally but not axially offset from each other. This offset is achieved, for example, by rotating one of the honeycombs about the common flow axis at a rotation angle such that at least a majority of the channels in the first catalyst section open or empty at the outlet end thereof onto at least one channel wall section and at least two adjoining channel openings at the inlet end of the second catalyst section. Similar results can be achieved by laterally displacing one of the honeycombs with respect to the other in a direction transverse to the flow axis.
With either rotational or lateral honeycomb channel displacement, the channel offsets thus provided will result in channels in the first catalyst section emptying onto two or more adjoining channels in the second catalyst section. In all cases, however, the divided feed streams from the channels in the first section are constrained by the contacting relationship between the honeycombs to flow only into a limited set of adjoining channels in the second section. In this way the unguided flow and uncontrolled feed stream mixing such as occurs within mixing chambers provided in conventional catalyst beds are entirely avoided.
In a second aspect the invention may be seen to reside in an improved method for treating a two-phase gas-liquid feed stream with solid structured monolithic catalysts of honeycomb configuration. That method utilizes a reactor with two or more honeycomb sections such as above described to more efficiently process the gas-liquid feed stream.
In utilizing such a reactor to carry out the improved method, a gas-liquid feed stream passing through the catalyst bed within the reactor is directed past the inlet end of the first section of catalyst and into the plurality of honeycomb channels therein, this step effecting a division of the feed stream into a plurality of feed stream portions traversing the plurality of channels. Each of these feed stream portions is then reacted against the catalyst-containing channel walls of the channel as it traverses the first section of catalyst, and is discharged from the outlet end thereof as at least partially reacted feed stream portion.
The feed stream portions discharged from the channels of the first honeycomb section are not collected and recombined into a single feed stream for mixing, as in a conventional mixing section, but are instead conveyed directly into the second catalyst section. At that point, due to the lateral channel offset provided between the first and second honeycomb sections as above described, each of at least a majority of the feed stream portions emptying into the second catalyst section will be separated into smaller, subdivided feed stream portions. Where the honeycomb sections are in direct contact this separation will be caused by impingement of each feed stream portion upon one or more web segments forming the channel walls of the second section. The subdivided feed stream portions then pass with the subdivided feed streams from other upstream channels into the channels of the second honeycomb section for reaction against the channel walls thereof.
Apparent from the foregoing description is the fact that a reactant mixture flowing from the first catalyst section into the second catalyst section will be conveyed into the second section without any large-scale mass transfer of reactants in directions transverse to the common flow axis of the honeycombs. That is, since each of the feed stream portions that is subdivided at the inlet face of the second honeycomb section is constrained to empty only into a small collected set of channels (generally two, three or four adjoining channels) in the second catalyst section, only limited and controlled levels of turbulence and lateral intermixing will occur.
After passage through, and reaction within, the channels of the second catalyst section, the reacted feed streams may be merged into a collected stream for recirculation through the catalyst bed or discharged from the reactor. In axial flow arrangements, the collected stream may be further processed through other treating stages in downstream sections of the reactor prior to discharge, if desired.
Catalytic conversion processes carried out as above described are both economic and surprisingly efficient. An important advantage of this process is that levels of turbulence and intermixing effective to increase catalytic reactor efficiency are achieved without the need for separate flow distributors or mixers between the honeycomb catalyst stages within the reactor. Thus a good initial distribution of reactants into the first honeycomb section can be maintained throughout the entire length of following catalyst sections, if the sections are arranged in flow-connected relationship as hereinabove described.